i have a 2000 explorer with a 4.0 sohc i got a code for a bank 1 to lean first choice for replacement is the O2 sensor after replacing that i ubruptly got a bank 2 to lean so i replaced the O2 sensor on that side. right after that it went back to a bank 1 to lean. ive replaced the map sensor already a couple months ago and i cannot find any leaks what are the other possible problems. problem started the day after a long trip.